A deep inelastic reaction using a 460 MeV 82Se beam incident upon a thick 192Os target was performed at the Legnaro National Laboratory, Italy. The resulting γ-decays were measured using the GASP array. Results for 194Pt extend the known level scheme of the yrast structure from spin I = (12 ħ) to (20 ħ). The irregularities in the sequence of the new transition energies and total Routhian surface calculations show a breakdown in collectivity with an yrast oblate shape remaining to high spin.Rubio Barroso, Berta, Berta.Rubio@ific.uv.e

Axiotis, M.

Bazzacco, D.

Bizzeti, P.G.

Brandolini, F.

Broda, R.

Bucurescu, D.

De Angelis, G.

Farnea, E.

Gadea Raga, Andrés

Gelletly, W.

Ionescu Bujor, M.

Iordachescu, A.

Jones, G.A.

Kröll, Th.

Langdown, S. D.

Lunardi, S.

Marginean, N.

Martínez Pérez, Trinitario

Medina, N. H.

Podolyák, Zs.

Quintana, B.

Regan, P. H.

Rubio Barroso, Berta

Schunck, N.

Ur, Calin A.

Valiente Dobón, J. J.

Walker, P. M.

Williams, S. J.

Zhang, Y. H.

G. A. JONES

Z.S. PODOLYAK

N. SCHUNK

P. M. WALKER

G. DE ANGELIS

Y. H. ZHANG

M. AXIOTIS

D. BAZZACCO

P. G. BIZZETI

R. BRODA

D. BUCURESCU

E. FARNEA

W. GELLETLY

A. GADEA

M. IONESCU BUJOR

A. IORDACHESCU

T.H. KROLL

S. D. LANGDOWN

N. MARGINEAN

T. MARTINEZ

N. H. MEDINA

B. QUINTANA

P. H. REGAN

B. RUBIO

C. A. UR

J. J. VALIENTE DOBON

S. J. WILLIAMS

BRANDOLINI, FRANCO

LUNARDI, SANTO

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Oblate Collectivity in the Yrast Structure of 194Pt

A deep inelastic reaction using a 460 MeV 82Se beam incident upon a thick 192Os target was performed at the Legnaro National Laboratory, Italy. The resulting γ-decays were measured using the GASP array. Results for 194Pt extend the known level scheme of the yrast structure from spin I = (12 ħ) to (20 ħ). The irregularities in the sequence of the new transition energies and total Routhian surface calculations show a breakdown in collectivity with an yrast oblate shape remaining to high spin

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Vol. 36 (2005) ACTA PHYSICA POLONICA B No 4OBLATE COLLECTIVITY IN THE YRASTSTRUCTURE OF 194Pt∗G.A. Jonesa, Zs. Podolyáka, N. Schuncka, P.M. WalkeraG. De Angelisb, Y.H. Zhangb, M. Axiotisb, D. BazzaccocP.G. Bizzetid, F. Brandolinic, R. Brodae, D. BucurescufE. Farneab, W. Gelletlya, A. Gadeab, M. Ionescu-BujorfA. Iordachescuf , Th. Kröllb, S.D. Langdowna, S. LunardicN. Margineanb, T. Martinezb, N.H. Medinag, B. QuintanahP.H. Regana, B. Rubioi, C.A. Urc, J.J. Valiente-Dobónaand S.J. WilliamsaaDept. of Physics, University of Surrey, Guildford, GU2 7HX, UKbINFN, Legnaro National Laboratories, Legnaro, ItalycDipartimento di Fisica and INFN, Padova, ItalydDipartimento di Fisica and INFN, Firenze, ItalyeThe H. Niewodniczanski Institute of Nuclear Physics PAN, Krakow, Polandf Institute of Physics and Nuclear Engineering, Bucharest, RomaniagInstituto de Física, Universidade de São Paulo, São Paulo, BrazilhUniversity of Salamanca, SpainiInstituto di Fisica Corpuscular, Valencia, Spain(Received November 30, 2004)A deep inelastic reaction using a 460 MeV 82Se beam incident upona thick 192Os target was performed at the Legnaro National Laboratory,Italy. The resulting γ-decays were measured using the GASP array. Resultsfor 194Pt extend the known level scheme of the yrast structure from spinI = (12 ~) to (20 ~). The irregularities in the sequence of the new transi-tion energies and total Routhian surface calculations show a breakdown incollectivity with an yrast oblate shape remaining to high spin.PACS numbers: 23.20.Lv, 27.80.+w, 25.70.Hi1. IntroductionHeavy neutron-rich nuclei with A ∼ 190 are exciting research areas formany nuclear structure phenomena including K-isomerism, γ-softness andoblate-prolate shape change. Little spectroscopy of neutron-rich nuclei in∗ Presented at the XXXIX Zakopane School of Physics — International Symposium“Atomic Nuclei at Extreme Values of Temperature, Spin and Isospin”, Zakopane,Poland, August 31–September 5, 2004.(1323)1324 G.A. Jones et al.this mass region has been performed. Using different reaction techniquessuch as projectile fragmentation, or deep inelastic reactions combined withsensitive γ-ray spectrometers, it is now possible to access this region andgain valuable information.2. Experimental details and resultsA range of nuclei in the A ∼ 190 region were excited using deep inelas-tic multi-nucleon transfer reactions, and their subsequent γ-decays detectedwith the GASP array at the Laboratori Nazionali di Legnaro, Italy. A82Se beam at an energy of 460 MeV impinged upon an isotopically enriched(97.8%) 50 mg/cm2 192Os target backed with 0.2 mm of 181Ta to stop allreaction products.Previously unknown γ-ray transitions have been identified for beam-like[1] and target-like [2] fragments. Fig. 1(a) shows a spectrum of transitionsassociated with 194Pt. Highlighted (#) are transitions in this nucleus thatFig. 1. (a) γ-ray spectrum highlighting the transitions in 194Pt double gated on allpreviously known yrast transitions from 0+ → (12+) where ∗ and # denote doubletand new transitions, respectively. (b) Partial level scheme for 194Pt. (c) TRS plotsfor (i) I = 14 ~ with an energy minimum offset from the oblate collective axis anda small deformation of β2 ∼ 0.08; (ii) I = 16 ~ with an oblate collective minimumat β2 ∼ 0.15.Oblate Collectivity in the Yrast Structure of 194Pt 1325have been observed for the first time. The 619 keV transition must be treatedas tentative due to it being unresolved from the first-excited transition inthe binary partner 78Ge. There is evidence for the previously observed[3] 652 keV transition despite the intense 655 keV transition from the firstexcited state of the 82Se projectile. Due to the dominance of the 655 keV, andthe weakness of the 652 keV transitions, it was not possible to confirm thecoincidence of the 652 keV with the 290 keV transition, and the latter shouldthus be treated as tentative. The 656, 183, 255 and 592 keV transitions are allin coincidence with the lower portions of the ground state and negative paritybands. Table I contains relative γ-ray intensities, total transition intensitiesand tentative spin assignments for the new states shown in the revised levelscheme of Fig. 1(b). The ordering of the new transitions was established byanalysing a variety of double-gated spectra and the relative γ-ray intensitieswere calculated using a double-gated spectrum of the 2+ → 0+, 328 keV and4+→2+, 483 keV transitions. The total transition intensities are calculatedon the assumption of stretched E2 transitions for the 656, 183, 255, and592 keV γ-rays. DCO ratios and angular distribution analysis could notdetermine unambiguously the multipolarities of the new transitions due totheir weak intensities. If dipole multipolarities are considered for the fournew yrast transitions above 12 ~, the ordering remains the same, althoughthe intensities of the 183 and 255 keV γ-rays are brought to within onestandard deviation, and it is thus increasingly possible that their orderingis reversed.TABLE Iγ-ray energies, γ-ray intensities Iγ , total transition intensities IT and level assign-ments for the new transitions (#) and B(E2) values of transitions from the 10+states in 194Pt (847, 749 keV [4], 338.8 keV [5]) measured in Weisskopf units.Eγ(keV) Iγ IT (E2) Ei → Ef (keV) Jpii → Jpif B(E2)183.0# 42 (5) 64 (8) 3670→ 3487 (16+) → (14+)225.0# 41 (5) 3057→ 2438 → (10+)255.0# 47 (5) 54 (5) 3925→ 3670 (18+) → (16+)290 # 21 (4) 23 (4) 2990→ 2700 (13−) → (11−)592.1# 48 (6) 49 (6) 4517→ 3925 (20+) → (18+)619.4# 92 (7) 2663→ 2438 → (10+)657.6# 91 (7) 92 (7) 3487→ 2829 (14+) → (12+)338.8 100(11)a 107(12) 2438→ 2099 10+ → 8+ 0.17(3)749 2848→ 2099 10+ → 8+ 34 (6)847 2916→ 2099 10+ → 8+ 42 (+8−10)Note: a The γ-ray intensities are normalised to 100 units for the 338.8 keV transition.1326 G.A. Jones et al.3. DiscussionPrevious experiments [3, 4] have identified the yrast structure of 194Ptup to I = (12 ~), which is extended to (20 ~) in the present work. In theCoulomb excitation work of Ref. [4], two Ipi = 10+ states close in energyto the yrast 10+ state were identified — a rotationally aligned pi(h11/2)−2state, and a member of the ground state band. The T1/2 = 6.4 ns [6] isomericyrast 10+ state is attributed to a ν(i13/2)−2 structure [4]. B(E2) values forthese states can be seen in Table I. The B(E2) of the yrast 10+ isomeris approximately a factor of 20 smaller than those of the transitions fromthe two other 10+ states. This indicates a significant change in structurecompared to the 8+ member of the ground state band.Total Routhian Surface (TRS) plots have been performed for 194Pt usinga technique identical to that of Ref. [7]. The parameterisation of Ref. [8]was used for the macroscopic part of the total energy. The TRS plots shownin Fig. 1(c) for spins 14 ~ and 16 ~ represent the yrast states of the posi-tive parity of the nucleus. The calculations show that the nucleus is oblatein its ground state and its yrast structure up to 8 ~ is dominated by anoblate collective minimum with β2 ∼ 0.14. They show that the nucleusdoes not exhibit purely collective oblate behaviour between 8~ and 14 ~,which is qualitatively consistent with the transition energies and the ex-pected change in structure indicated by the 10+ isomer. The calculationsshow a return to oblate collective rotation at 16 ~ and 18~, only to exhibitagain a non-collective or triaxial nature at 20 ~, also consistent with thetransition energies at these spins.The observed yrast structure of 194Pt in the I = (12~) → (20 ~) regioncontrasts with the systematics of its isotonic and isotopic partners [3, 9],which have typical rotational yrast structures with an oblate-prolate shapechange at high spins. However the TRS calculations for 194Pt show an oblateshape that, unusually, persists to 20 ~. The oblate shape of 194Pt at lowspin is substantiated in the Coulomb excitation work of Ref. [4]. A similarpersistance of oblate shapes to high spin has also been predicted in theneutron-rich A ∼ 110 region [10].REFERENCES[1] Y. H. Zhang et al., Phys. Rev. C70, 024301 (2004).[2] Zs. Podolyák et al., Int. J. Mod. Phys. E13, 123 (2004).[3] S.A. Hjorth et al., Nucl. Phys. A262, 328 (1975).[4] C.Y. Wu et al., Nucl. Phys. A607, 178 (1996).[5] E. Browne et al., Nucl. Data Sheets 79, 277 (1996).Oblate Collectivity in the Yrast Structure of 194Pt 1327[6] H. Ton et al., Nucl. Phys. A153, 129 (1970).[7] T.R. Werner, J. Dudek, At. Data Nucl. Data Tables 59, 1 (1995).[8] K. Pomorski, J. Dudek, Phys. Rev. C67, 044316 (2003).[9] R.B. Firestone et al., Table of Isotopes, Eigth Edition, Vol. 2, 1996.[10] F.R. Xu et al., Phys. Rev. C65, 021303 (2002).

Oblate collectivity in the yrast  structure of 194Pt

Oblate collectivity in the yrast structure of 194Pt

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